[49.06] Triton's Atmospheric Structure in 2001 from the Tr231 Occultation

On 2001 August 25, an occultation by Triton of the star
designated Tr231 (McDonald and Elliot, Astronomical Journal,
119, 2000, 936-944) was observed from the 1.9-m telescope of
the South African Astronomical Observatory. Preliminary
analysis indicates that this light curve has the highest
signal to noise of any Earth-based observations of Triton
occultations (Olkin, et al., Icarus, 129, 1997, 178-201;
Elliot, et al., Icarus, 148, 2000, 347-369). Initial
inversions of this light curve indicate a thermal profile
with a positive thermal gradient (dT/dR) at altitudes above
~50-60 km, but having an isothermal or negative gradient
below. These results are qualitatively consistent with those
from the 1997 (Tr180) HST occultation light curve (Elliot,
et al., Icarus, 143, 2000, 425-428). The initial thermal
gradient of those results can be explained by the downward
conduction of heat deposited high in the atmosphere coupled
with CO radiative cooling. The addition of a currently
unidentified cooling mechanism at lower altitudes is
required to explain the isothermal or sub-isothermal profile
below 50km. Analysis of the Tr231 event is continuing. This
work is partially supported by NASA grants NAG5-10444 and
NAG5-9008.